1. Field of the Invention
This invention relates to article handling devices and more particularly to support means for moving a work piece into position. This apparatus is suitable particularly for positioning of semiconductor wafers during testing.
2. Description of Related Art
Testing of circuits on wafers involves supporting the chips in wafer form on a top plate on a support usually in the form of a single column, known as a Z stage. FIG. 1 shows a schematic diagram of a prior art type of Z stage 10 in which the entire Z stage 10 is carried by and secured to a support 11 below it, which moves in the X and Y directions and includes an elevator column 12 for moving a top plate 14 up and down, in the Z direction. The elevator column 12 comprises a single central supporting column in the form of an axle usually as the sole support located centrally under the top plate 14. The column 12 is supported for sliding lengthwise by bearing 13 which introduces friction caused by probing by probe 15, particularly near the edge of a wafer being tested, resting on the top plate 14 as well, when the probe 15 is testing near the periphery of the top plate 14. Bearing 13 is fastened to support 11. In addition, the probe 15 causes excessive deflection of the elevator column 12 when it contacts wafers, near the periphery of the top plate.
U.S. Pat. No. 4,534,695 of Stump, et al. "Wafer Transport System" shows a wafer supported by four quartz rods in a lift for an annealing system. It does not support the work at the periphery or for anything other than light treatment or deposition, since there is very little support for the wafer.
U.S. Pat. No. 4,558,984 of Garrett for "Wafer Lifting and Holding Apparatus" shows an elevator system for wafers.
U.S. Pat. No. 4,560,880 of Petric et al., for an "Apparatus for Positioning a Workpiece in a Localized Vacuum Processing System" shows a Z-stage supported above a set of three hydraulic actuators 28 in FIGS. 1 and 3. The actuators 28 are bellows which are pressurized by a hydraulic controller 70 which is driven by a linear stepper motor 80 which adjusts the pressure to all three of the actuators as a function of feedback from a gap sensor. There are three spring assemblies 94 composed of a flanged pin 98 extending down from the Z stage and a spring in a cup 96 on the X-Y table. In addition, there is a central post 86 which is attached rigidly to the X-Y table 56 and a flexible sheet 88 secured to the Z stage.
U.S. Pat. No. 4,528,451 of Petric et al. for "Gap Control System for Localized Vacuum Processing" describes a wafer handling system for a particle beam system, E-beam or ion-beam lithography systems or an ion implanter. Several piezoelectric devices are used to provide Z-motion of the wafer. A piezoelectric element 76 is mounted at one end to a support bracket 78. The opposite end of the element 76 bears against one arm of a lever 80. The other arm of the lever 80 is coupled through a pin 82 to a vacuum chuck 18. The lever 80 pivots about a flexible web 84 connecting the lever 80 to a support block 86. The material of the flexible web 84 can be beryllium copper. The lengths of the arms of the lever 80 can be designed to provide the required amount of z-axis motion. The support block 86 and the support bracket 78 are mounted to the stage 24. When the piezoelectric element 76 is energized by an electrical signal, it increases or decreases in length. The change in length is converted by the lever 80 into vertical, or z-axis, movement of the vacuum chuck 18. Alternatively, a Burleigh "inch worm" piezoelectric element can be utilized.
The drives of Petric et al patents are not as strong as a hydraulic drive and the linkages are not as simple or as coordinated as is desirable.